Folded Plate Structures

This article was originally published in Blue 02: Systems and Structure in 2010. You can see the article in its original format, and other articles, online.

Horno3: Museo Del Acero in Monterrey, Mexico, is comprised of two major elements: the restoration of a derelict 1960s blast furnace complex (recently designated as a National Industrial Heritage Site) and a new addition providing gallery space and other museum facilities.

The museum showcases the history of steel manufacturing, with exhibits as well as a pyrotechnical show; it also chronicles the industrial history of the state of Nuevo Leon and its capital Monterrey, which for much of the 20th Century was renowned for its steel production.

The architectural challenge was to balance sensitive historic preservation against the requirement for a new purpose for the complex as a dynamic symbol within its changed context – the surrounding steelworks having been converted into a public park. The building was completed in September 2007 when the city hosted the Universal Forum of Cultures.

The restored blast furnace complex comprises of an 80m high Blast Furnace, a ‘Cast Hall’ and an array of supporting facilities. Adjacent to the Furnace, located on the site of a former slag-heap, the new addition houses an entry wing, the main public stairs and an interactive gallery space: the Steel Gallery, which acts as an architectural focal point and a complementary component to the existing industrial elements. In order to work discreetly within the context of the blast furnace, this new gallery is largely subterranean, situated underneath a green roof that gently slopes down to the surrounding public park.

Both the refurbishment and new additions, interventions, etc. respond strongly to the site’s history as a steelworks. This is made most explicit through a series of structural elements, including a single-span helical steel stair, a façade system and the folded – or more accurately, tessellated – Steel Gallery Roof.

The architectural challenge was to balance sensitive historic preservation against the requirement for a new purpose for the complex as a dynamic symbol within its changed context.

For this important element we were striving to achieve the following ambitions:

- To ensure the roof functioned didactically, clearly expressing the structural forces present, as well as demonstrating the engineering and construction possibilities afforded by steel – serving as an exhibit in its own right

- To achieve structural efficiency and utility, using material strength, structural design approach, connection and building-technology.

- To utilise geometry that simultaneously responded to planning constraints and incorporated other important considerations such as rainwater collection.

- To function architecturally for both spaces, above and below – the former manifested as a planted roof landscape open to the surrounding park, and as the ceiling and container of the Steel Gallery below.

With those goals in mind, it became clear early on that we wanted to demonstrate the structural possibilities and capabilities of steel in a unique and rather unusual way, from an architectural but even more so an engineering point of view. Rather than the common post and beam/truss system where the steel members are all linear elements, we sought to use steel plates, to create a structurally performative surface. All those plates are planar triangles; this means no bending or complicated distortion was necessary. This initiative corresponded flawlessly with the idea of introducing folds into the surface to create the demand for rigidity that a structure of this scale requires, but without adding any further mass. We merged the commonly separated skin and bones of a building into a single form.

A space resembling a geode served as a further analogy; a geometrically precise, yet hidden interior, formed as a result of imposed external forces, with a rough and more naturalistic exterior. As this building merges with the landscape, we perceive a hidden, subterranean jewel-like space nestled within the natural surroundings of the green roof and adjacent to a raw steel wall which serves to delineate it from the other museum zones. This space is meant to be discovered and revealed to visitors as they journey through the museum.

On a project-wide scale a more formal relationship exists between the circular nature of the existing supporting facilities arrayed around the blast furnace and the newly constructed helical stair and rounded Steel Gallery, placed to draw from and reinforce this organisation. Functionally, the circular space works well with the exhibit design’s programmatic necessity of a central performance space comprising a stage and tiered seating. Mainly interactive exhibits are arranged radially around this performance space, along the visitor's path of circulation. To create a more dynamic space and satisfy differing exhibition requirements, the performance space was designed and located asymmetrically.

With the planning, basic-form, and structural concept in place, we started to develop the specific geometry of the roof. With the help of a parametric computer model, a set of geometric relationships was established, allowing adaptive structural shaping and responsiveness to further constraints as the design evolved. Examples of these additional functional and spatial constraints are the minimum slopes to assure effective drainage and clearance below the bridge, which connects the terrace with the Cast Hall.

In order to achieve a load-bearing and self-supporting structure, with the largest span of 13m (43ft) and a diameter of 30m (98ft), the design relied on extensively calculated stress analyses which allowed the optimisation of the design and the advance of the known engineering limits of structural steel. With the use of a 3D finite element model, deformations and stress peaks were evaluated as well as plate buckling limits taken into consideration; these limits were found; these limits ultimately proved to be the main structural driver for the roof design.

To address local buckling and varying spans as well as to minimise stresses, the structural heights and plate thicknesses were adjusted iteratively. With a maximum fold-depth of 1.5m (~5ft) the steel plates could be kept relatively thin, thicknesses vary between 10 and 20mm; column plates have a thickness of 13mm. Additional folds were added to the geometry where increased plate thickness alone would not suffice; further beams or stiffeners were not required throughout the entire structure. As the spans increase, the depth of the folds also gradually increases. This is clearly visible from underneath.

Within the central ring of columns and the series of radiating roof segments, there exist compressive forces which push to the perimeter. These forces are offset by a closed tension ring system which works due to the roof’s circular nature, i.e. the structure is in equilibrium.

Additional elements were designed independently of the parametric model, but their implications were recorded in it. The 12 main structural support columns for example, were initially considered as single plates with one fold, but to provide the required structural rigidity and movement connections at the top, a multi-faceted and hollow profile that transitions or 'lofts' from a chevron to a triangle was developed. This allows for column-integrated roof drainage and sprinklers to keep the underside free of any surface-mounted infrastructure.

To incorporate architectural lighting as well as theatrical light fixtures for the stage, we designed a circular boom: a trough with a single fold, cantilevered inwards from the inner column ring.

In a similar fashion, tapering chevron-cantilevers reach out to support the terrace which hovers over the folded roof, separated by a ring of glass-blocks to distinguish the two elements architecturally. The terrace structure, sitting with omni-directional sliding joints on those cantilevers, is laterally restrained through the bridge to the main building.

Along with the usual set of drawings, the steel fabricators received a simple 3D model of the finalised geometry containing only surfaces with zero thickness. To those surfaces they had to apply the required thickness, inwards in the case of the columns and upwards at the roof segments to maintain the architectural intent.

During collaborative workshops we also developed a clear methodology of prefabricating all 24 segments and 12 columns in the shop as well as the assembly sequence on site.

This approach was validated by the construction of several essential models at full, ½ and ½ scale. Through this process the fabricator gained important experience in plate assembly and welding procedures which proved invaluable when it came to achieving consistent edges with sharp angles. Furthermore, the mock-ups served to test various means of on-site erection and the final surface finish.

As a further advantage to shop-fabrication all elements were inspected before being shipped to site as part of a stringent quality management regime. All these efforts ultimately enabled increased accuracy as well as the minimisation of on-site welding and the reduction of installation time. The final structure behaves like monolithic folded surface as its prefabricated segments are continuously and fully welded and the craned-in segments continuously bolted.

The landscaped berms, in addition to the roof around the Steel Gallery, are vegetated and predominantly planted with native Love Grass, which is a species adapted to the low rainfall typical of Northern Mexico. The Steel Gallery roof itself is planted with a quilt of flowering sedums that follow the form of the tessellated structure below. In all cases the vegetation has been specifically selected to reduce maintenance and irrigation, as well as being designed to provide habitats for local forms of wildlife and a resting spot for the annual butterfly migration that passes through the area.

This absolutely performance-based design demonstrates how, with today’s computer-aided technology, steel as a sheet material is able to be transformed into structurally rigid forms by complex folding. From the outset, using this technique required close co-ordination and collaboration between the structural engineers (Werner Sobek), the Mexican fabricator (Paileria San Luis Potosi) and the Grimshaw team. We are delighted that the end result, as well as being a very rewarding iterative process, also, enabled the creation of a roof that serves manifold architectural and structural functions with the deployment of a highly specific geometry, which is complex without being complicated.